In the case of deep-water production, one of the factors which most affects the flow of petroleum is the hydrostatic pressure which is due to the level difference existing between the wellhead and the production platform. The hydrostatic pressure, depending upon the situation, may represent up to more than 90% of the pressure drop between the wellhead and the platform, and therefore it is necessary for the petroleum to be pumped.
Many proposals have been presented for the purpose of defining the method for the flow of such petroleum, with perhaps the simplest method being subsea separation of oil and gas, and their single-phase flow up to the nearest platform. This separation system has the characteristics required for deep-water installation and a motor-pump to allow the oil to flow, thereby overcoming the high pressure (hydrostatic pressure drop) characteristic of this type of application.
The conventional production systems of oil fields of natural flowing usually include wells, individual flowing lines, manifolds, additional lines, and, eventually, risers in case of offshore installation. The fluids produced, usually in the form of a multiphase mixture of oil, gas and water, pass through all of the components of the production system, up to a separation vessel installed at the gathering station (on-shore) or at the production platform (off-shore).
The individual well production is a direct function of the pressure drops from the reservoir rock to the separator. Therefore, if the pressure at the separator is maintained high, or if the pressure drops along the piping are large, the production rates of the wells will be small. This is because the only form of energy for moving the mixture is the pressure of the reservoir itself.
The offshore production systems usually utilized in shallow waters aim at minimizing the pressure drops. This is accomplished by minimizing the lengths of the flow lines and the riser, thereby optimizing the production level of the wells. This is usually achieved by locating the production platform (with the pumping and processing systems) directly on the area of the reservoir.
For oil fields located in deep waters (above 400 m), the positioning of the platform directly on the reservoir is a difficult operation, since it requires the utilization of huge fixed structures or complex floating structures which are prohibitive from both technical and economic points standpoints.
Despite continued research work concerning the positioning of the platform directly on the reservoir, various other production alternatives have been considered. Among the most promising are those which make use of well completion with wet Christmas trees and a pumping system capable of adding energy to the fluids produced, with the purpose of transporting them to a production platform located in shallower waters or directly onshore.
The technical difficulty of this last production alternative resides in the pumping system, which must work with high pressures and flow rates of multiphase mixtures. The use of these multiphase pumps is based on the necessity of low-pressure maintenance at the wellhead to ensure an adequate production level.